The invention relates to a propulsion device particularly for a spacecraft, comprising a plasma chamber in which a plasma is producible in a propellant, and a focussing arrangement for focussing an electromagnetic radiation field into a focussing region in the plasma chamber in order to produce the plasma.
It also relates to a method of forming shock waves, particularly for propelling a spacecraft, by producing a plasma in a propellant, wherein an electromagnetic radiation field is focussed into a focussing region of a plasma chamber to produce plasma in the propellant.
It has been proposed for example in U.S. Pat. No. 3,825,211 to propel a spacecraft by means of a laser beam, the laser being installed on earth or in a satellite. For this purpose the spacecraft has a parabolic cylindrical reflector, and a propellant is fed to a focal axis of that reflector.
Similar arrangements are described in U.S. Pat. Nos. 3,818,700, 4,426,843 and 5,152,135.
The laser beam produces a plasma in the propellant, which expands and thus in turn accelerates the propellant. This is then converted into propelling energy to propel the spacecraft by a suitable device such as a nozzle arrangement.
With the development of pulsed high-powered lasers the high spatial coherence of laser radiation enables the radiation energy to be transmitted into the plasma chamber over distances of the order of magnitude of 100 to 1000 km.
As a laser-assisted propulsion system need not carry its energy generator with it and may even be installed quite far away from the propulsion system, the starting mass of a spacecraft provided with such a propulsion system can be kept low. In addition higher specific momentums can be produced with laser propulsion systems than with conventional propulsion systems based on chemical combustion.
It is the object of the invention to provide a propulsion device and a method of forming shock waves, which is highly efficient and controllable, respectively.
This object is achieved in accordance with the invention, in that there is arranged in the plasma chamber a plasma ignition arrangement for bringing a material (the ignition medium) which lowers the plasma breakdown threshold in the propellant into the focussing region.
In accordance with the invention it is possible to produce a plasma at a pre-defined location in the focussing region, namely substantially where the material which lowers the plasma breakdown threshold is positioned or introduced, the plasma in turn leading to the formation of a pressure wave. Plasma ignition thereby takes place in a controlled manner and particularly at a point-like location, that is to say, the time and place of ignition are guaranteed to be highly reproducible.
As the electromagnetic radiation field and particularly laser radiation is absorbed better by a plasma than by a neutral propellant, the defined plasma production also increases propelling efficiency; the invention greatly improves the coupling in of the energy of the electromagnetic radiation field into the propulsion device. In particular the invention makes it unnecessary to feed an additional absorber medium to the plasma chamber as proposed e.g. in U.S. Pat. Nos. 3,818,700 or 4,036,012.
The fact that the plasma and consequently plasma pulses are produced at a pre-defined location in the plasma chamber, owing to the introduction of the material which lowers the plasma breakdown threshold in the propellant into the focussing region, makes it possible to control the thrust vector of the propulsion device, as the pre-defined location of plasma production in the focussing region is controllable. This can be achieved in a simple way by positioning the threshold-lowering material in the focussing region in a controlled manner. The position of a spacecraft provided with the propulsion device in accordance with the invention can thereby also be controlled.
With the location of plasma formation being controllable it is also possible to inject the electromagnetic radiation field at a transverse angle of incidence to the axis of the plasma chamber and thus to inject it transversely to the axis of a spacecraft, as the thrust vector of the propulsion device can accordingly be appropriately tilted by controlling the pre-defined location of plasma production. It is then unnecessary for the focussing arrangement to be aligned accurately with the incident electromagnetic radiation field. As a result the coupling of the energy of the electromagnetic radiation field into the plasma chamber can be improved, as the coupling field can be correspondingly guided; it may e.g. be advantageous for the radiation field not to be guided through a propellant discharge region of the drive device in order to avoid energy losses through absorption.
Misfiring is largely avoided by use of the material which lowers the plasma breakdown threshold in the propellant. Such misfiring starts in particular at a wall of the plasma chamber and leads to degradation and/or denudation of material in that structure. Moreover use of an ignition medium greatly reduces fluctuations in the magnitude and direction of the thrust vector of the propulsion device, which are observed without such a medium.
It is especially advantageous for the material which lowers the plasma breakdown threshold to be a metal. In particular the material which lowers the plasma breakdown threshold is copper. For example the breakdown threshold in air has been found to be about three orders of magnitude higher than the corresponding value for copper vapour over a copper surface in air. In the case of a CO2 laser with a wavelength of 10.6 xcexcm given a pulse duration of 10 xcexcs the breakdown threshold in air is about 15 kJ/cm2 and that in copper vapour about 10 J/cm2. In this connection see also H M Musal, xe2x80x9cPulsed laser initiation of surface plasma on metal mirrorsxe2x80x9d, Bennett, H E, Glass, A J, A H Guenther, eds, Damage in laser materials: 1980, Nat. Bur. Stand. (U S) Spec. Publ. 620 (1981), page 227.
It is particularly advantageous for the plasma ignition arrangement to be arranged in the plasma chamber so displaceably that the material which lowers the plasma breakdown threshold can be positioned within the focussing region. In this way a plasma pulse can be produced at a reproducible, pre-defined location in a controlled manner within the plasma chamber, thereby allowing thrust vector control of the drive device. Such thrust vector control enables an angular momentum to be generated, as a means of changing the position e.g. of a missile. The corresponding change in the angular momentum of the missile depends on how the resultant thrust vector is located relative to the centre of gravity of the missile. Rotating movements of the missile about an axis transverse to the direction of flight can then also be obtained by changing the mass distribution in the missile with no change in the resultant thrust vector. Since the thrust vector is controllable according to the invention, i.e. it can in particular also be maintained constant, there is thus a further method of changing the position e.g. of a missile.
In a particularly advantageously designed embodiment the plasma device includes a pin made of a material which lowers the plasma breakdown threshold. This pin then acts as an ignition pin. It can be positioned in the plasma chamber and especially in the focussing region in a simple manner to control the thrust vector of the propulsion device (e.g. in order to adjust the position of a spacecraft). The actual pin material may be used as a fuel when the pin evaporates; the material vapour can then act as an absorber to improve plasma production and also directly help to generate thrust. Plasma is produced in a substantially defined manner on a surface of the pin. Contact between the propagating plasma and an inner wall of the plasma chamber is thereby avoided.
It is beneficial for the pin to be arranged pivotably in the plasma chamber in order to control the thrust vector and thus the position e.g. of a spacecraft. Positioning the pin so that it can pivot within the focussing region in the plasma chamber enables the resulting pressure wave to be controlled.
The pin is beneficially displaceable in its longitudinal direction in the plasma chamber. This makes it possible for the material which lowers the plasma breakdown threshold (the ignition medium) to be followed up, particularly according to the burning action of the pin, as a means of ensuring uniform, reproducible formation of plasma pulses in the chamber.
It is structurally advantageous for the pin to be mounted in a sleeve which holds it in the plasma chamber. The sleeve provides a simple way of making the pin longitudinally displaceable, and if the sleeve is mounted for pivoting movement the pin can then be pivoted in the plasma chamber.
In an alternative embodiment a material which lowers the plasma breakdown threshold in the propellant may be blown into the focussing region. The threshold is then lowered at a blowing-in point at which the fluid ignition medium passes into the focussing region. The plasma ignition arrangement then advantageously comprises a member for blowing a threshold-lowering material into the focussing region. Provision is particularly made for the blowing-in member to be arranged pivotably in the plasma chamber, thereby allowing the location where the ignition medium is blown into the focussing region to be changed.
To enable the thrust vector to be controlled as required, an alternative embodiment provides for the blowing-in pressure of a material which lowers the plasma breakdown threshold to be adjustable into the focussing region. This particularly makes it possible to control the extent to which ignition medium blown into the plasma chamber from an orifice of the blowing-in member penetrates into the focussing region.
It is particularly advantageous for the electromagnetic radiation field to be introduced in to the plasma chamber in pulsed form. Reproducible, uniform plasma pulses can then be produced in the plasma chamber, resulting in shock waves which generate a thrust to drive e.g. a spacecraft.
It is beneficial when a laser pulse length is at most of the same order as the time taken by the plasma to propagate from its location of ignition to a geometric focal point. It is especially beneficial for the length of a laser pulse to be shorter than the corresponding propagation time. This guarantees that the plasma will ignite at the location defined by the plasma ignition arrangement and propagates from there as shock waves.
The electromagnetic radiation field is in particular a laser radiation field. High power can be fed into the propulsion device by means of pulsed, high-powered lasers, and transmission over distances of the order e.g. of 100 km to 1000 km is possible owing to the spatial coherence of laser radiation. Thus energy from a ground station may be fed into the focussing arrangement over great altitudes, to propel a flight vehicle provided with the propulsion device according to the invention.
In an alternative embodiment with a favourable manufacturing method the plasma chamber itself is constructed as focussing arrangement. No additional focussing arrangement need then be provided to introduce the electromagnetic radiation field into the plasma chamber and effect focussing there. It is advantageous for a wall of the plasma chamber to be constructed as a reflector. Radiation passed into the chamber is then reflected by a wall of the chamber into a focussing region where it can ignite a plasma.
To obtain high reflectivity a wall of the plasma chamber is advantageously polished or coated with a reflective layer or metallized, so that it absorbs or scatters as little radiation power as possible and so that the greatest possible radiation power reaches the focussing region.
A plasma chamber may be conical or of a paraboloid shape. It may also comprise a plurality of segments to guide the electromagnetic radiation into a focussing region. A paraboloid shape for the chamber is favourable, as there is a large opening angle ready to introduce the electromagnetic radiation and also a correspondingly large opening angle for guiding out the pressure wave to propel the propulsion device. In addition the radiation can be concentrated substantially independently of the direction of incidence.
It is then beneficial for the plasma ignition arrangement to be held in the region of an apex of the plasma chamber. The igniter holder is thus substantially outside the beam path of the electromagnetic radiation field and is affected very little by the plasma and resultant pressure wave.
In a particularly advantageous alternative embodiment air, especially atmospheric air, is used as the propellant. The propellant need not then be carried, so the weight is reduced and the design of the propulsion device is kept simple. Air as a propellant is suitable for propulsion devices used in the atmosphere, such as propulsion devices used near Earth""s surface, for missiles within the atmosphere or for spacecraft as a propulsion stage within the atmosphere.
It is then beneficial for the plasma chamber to be provided with propellant feed apertures. These may in particular be slots arranged in a front region of the plasma chamber so that the propellant, especially atmospheric air, can easily pass into that chamber.
Alternatively or additionally a gas and/or liquid and/or solid carried by the propulsion device may be used as a propellant. If such propellants are blown into the plasma chamber in outer space a corresponding thrust can be achieved through the formation of plasma in the propellant. But in contrast with rockets propelled by chemical combustion processes an oxidant as required for combustion need not be carried.
It is beneficial for the electromagnetic radiation field to be used to evaporate the propellant. Part of the injected radiation power may for example be branched off for this purpose.
In an advantageous alternative embodiment a plasma chamber and/or a focussing arrangement is such constructed that there are a plurality of focussing regions provided. For example a plurality of channel-shaped paraboloids may be arranged adjacent each other. In this way a specific thrust vector can be set as required according to the particular application.
A plasma ignition arrangement is then beneficially associated with each focussing region, to obtain numerous possibilities of controlling the drive device.
In an advantageous alternative arrangement a plurality of plasma ignition arrangements are provided for one plasma chamber. According to the individual controlling of the various ignition arrangements pressure wave generation in the plasma chamber can then be controlled as required and a specific thrust vector thus generated and controlled according to the particular application.
It is beneficial if the plasma ignition arrangements can be individually controlled, e.g. if pins made of an ignition medium can be moved into or out of a focussing region. In this way a specific pressure wave distribution to generate a resultant thrust vector or to form a shock wave with a defined front can be adjusted in the plasma chamber.
In another alternative embodiment a plasma chamber has one or more focal lines. For this purpose reflectors of the focussing arrangement may for example be channel-shaped or toroidal. The plasma can then be ignited along such a focal line, thereby forming an appropriate pressure wave to generate a thrust vector for the propulsion device.
For example a focal line may be closed, with the corresponding reflector of the focussing arrangement in a toroidal shape. The plasma can then in particular be produced rotationally symmetrically to a centre of such a closed focal line.
It is beneficial for a plurality of plasma ignition arrangements to be arranged along a focal line in the plasma chamber, particularly so that the plasma can be ignited symmetrically on the focal line.
An alternative embodiment provides for a plasma ignition arrangement to comprise a blade-shaped member made of a material which lowers the plasma breakdown threshold in the propellant. Plasma ignition then preferably takes place at a sharp edge of such a member.
It is beneficial for a plasma ignition arrangement to be pivotable transversely to a focal line. The location of ignition can then be appropriately adjusted on the focal line. It is further beneficial for a plasma ignition arrangement to be height adjustable relative to a focal line, so that the place of plasma ignition can be specifically controlled.
The invention further relates to a shock wave generator wherein the propulsion device according to one of claims 1 to 30 is fixed stationary.
A generator of this type forms shock waves which may be used e.g. for testing material. In accordance with the invention the shock waves may be specifically controlled in respect of their propagation direction and the magnitude of their propagation vector.
The shock wave generator has the same advantages as those already explained in connection with the propulsion device according to the invention.
According to the invention the above-mentioned problem is further solved by the method described above, in that a material which lowers the plasma breakdown threshold in the propellant is brought into the focussing region.
This gives the advantages already explained in connection with the propulsion device according to the invention. Other advantageous embodiments have also been explained in connection with those propulsion devices.
It is beneficial for the material which lowers the plasma breakdown threshold to be positioned in the focussing region as a pin. This gives rise to many possibilities of controlling the pressure wave emerging from the plasma chamber. In particular the pin is movable relative to the focussing region for that purpose. The movement of the pin is advantageously controlled in order to control the discharge vector of the pressure wave, formed by the production of plasma, from the plasma chamber.
It is particularly advantageous for the electromagnetic radiation field to be supplied in pulsed form so that uniform plasma pulses can be produced in the plasma chamber, whereby pulsed pressure waves are formed.